"My guess is that the same-sex marriage issue being on the ballot puts California in play in McCain versus Obama," he said.
Land said various denominations and religious groups, including Mormons and Catholics, already were joining forces in California with evangelicals to defeat gay marriage.
"If California rejects their Supreme Court and changes their constitution, that's going to be huge...if we win in California we have a chance of winning this nationwide," he said.
No Republican presidential candidate has won California since George HW Bush in 1988. The most recent state polls show Obama leading McCain, generally by double digits, but most of those were conducted before the court ruling.
California, America's largest state and often a trend-setter for the rest of the country, will wed any gay couple, regardless of where they live. By contrast, Massachusetts, the other US state where same-sex couples can marry, only weds residents or those from a neighbouring state.
Land said this would put the issue on the evangelical radar screen outside of California.
"You're going to have same-sex couples getting married in California and coming back to all the states in the country and demanding that their marriages be recognised," he said. "So it's going to be an issue in Cedar Rapids, in Dallas, in El Paso."
California's size means it has the most electoral college votes up for grabs in the presidential race and the candidate who gets the most popular votes in a state wins all its electoral votes.
Many evangelical Christians, such as Southern Baptists, regard gay marriage as a "make or break" issue that threatens the traditional family and by extension their vision of a functioning society. They also see same-sex relations in general as sinful.
